Geological records from the Antarctic margin offer direct evidence
of environmental variability at high southern latitudes and provide
insight regarding ice sheet sensitivity to past climate change.
The early to mid-Miocene (23–14 Mya) is a compelling interval to
study as global temperatures and atmospheric CO2 concentrations
were similar to those projected for coming centuries. Importantly,
this time interval includes the Miocene Climatic Optimum, a period
of global warmth during which average surface temperatures were
3–4 °C higher than today. Miocene sediments in the ANDRILL-2A drill
core from the Western Ross Sea, Antarctica, indicate that the Antarctic
ice sheet (AIS) was highly variable through this key time interval.
A multiproxy dataset derived from the core identifies four distinct
environmental motifs based on changes in sedimentary facies, fossil
assemblages, geochemistry, and paleotemperature. Four major disconformities
in the drill core coincide with regional seismic discontinuities
and reflect transient expansion of grounded ice across the
Ross Sea. They correlate with major positive shifts in benthic oxygen
isotope records and generally coincide with intervals when atmospheric
CO2 concentrations were at or below preindustrial levels
(∼280 ppm). Five intervals reflect ice sheet minima and air temperatures
warm enough for substantial ice mass loss during episodes of
high (∼500 ppm) atmospheric CO2. These new drill core data and
associated ice sheet modeling experiments indicate that polar climate
and the AIS were highly sensitive to relatively small changes in atmospheric
CO2 during the early to mid-Miocene.